msm: kgsl: Fix context reference counting

Get rid of kgsl_find_context. Use instead kgsl_context_get that does
correct RCU read locking around the itr_find and increases the
reference count on the context before returning it.  This eliminates
the chance that a context will be destroyed while somebody is still
using it.  Of course increased use of kgsl_context_get is accompanied
by kgsl_context_put in all the right places.

Change-Id: Ic0dedbad73d497fd9b451aefad8e5b28d33b829d
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>